1. Field of the Invention
The present invention relates in general to a method of measuring optical characteristics of liquid crystal cells and its applications for measurement equipment and manufacturing method of liquid crystal devices. Specifically it is concerned with a method of measuring with precision the retardation of the liquid crystal cells and equipment to measure it.
2. Description of the Prior Art
A conventional method for measuring the retardation of a birefringent material such as a liquid crystal is disclosed, for instance in U.S. Pat. Nos. 4,973,163 or 5,239,365.
The invention disclosed in U.S. Pat. No. 5,239,365 relates to measurement of the retardation of a material having a uniaxial birefringence property, in which a sample to be measured is inserted between a polarizer and an analyzer both having their directions of polarization fixed at a specified angle with each other, a variation in the intensity of light transmitted through the polarizer, sample and analyzer is detected while rotating the polarizer and analyzer relative to the sample, rotation angles of the polarizer and analyzer are set based on the detected variation in the intensity of light and the retardation of the sample is calculated based on the detected variation in the intensity of light and wavelengths of light emitted from a light source. This method is limited to uniaxial birefringent materials and, accordingly, is not available to measurement of retardation of a liquid crystal cell such as a twisted nematic liquid crystal.
On the other hand, the invention disclosed in U.S. Pat. No. 5,239,365 provides a method for measuring the retardation of a liquid crystal cell such as a twisted nematic liquid crystal. This method comprises steps; placing a liquid crystal cell between a polarizer and an analyzer in an arrangement wherein the polarization angle of the polarizer is tilted by 45.degree. with respect to the orientation of the light entry surface of the liquid crystal cell and the polarization angle of the analyzer is tilted by 45.degree. with respect to the orientation of the light output surface of the liquid crystal cell; measuring the strength of light transmitted through the analyzer and calculating retardation d.DELTA.n based on the measured light strength. This method, however, is applicable only for a liquid crystal cell having known rubbing directions and, accordingly, is impossible to measure the retardation of a liquid crystal cell having unknown rubbing directions. Further, it is impossible to measure the retardation accurately in the case that there exists a variety in the rubbing direction of a liquid crystal cell.